Devices, systems and methods for reducing the concentration of a chemical entity in fluids

ABSTRACT

A device for removal of at least a portion of carbon dioxide from an aqueous fluid includes at least one membrane through which carbon dioxide can pass to be removed from the fluid and immobilized carbonic anhydrase on or in the vicinity of a first surface of the membrane to be contacted with the fluid such that the immobilized carbonic anhydrase comes into contact with the fluid. The first surface exhibits carbonic anhydrase activity of at least 20% of maximum theoretical activity of the first surface of the membrane based on monolayer surface coverage of carbonic anhydrase in the case that the carbonic anhydrase is immobilize on the first surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/811,265, filed Jun. 8, 2007, now U.S. Pat. No. 7,763,097, issued onJul. 27, 2010, which claims benefit of U.S. Provisional PatentApplication Ser. No. 60/811,810, filed Jun. 8, 2006, the disclosures ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to devices, systems and methodsfor reducing concentration of a chemical entity (for example, carbondioxide) in fluids and, particularly, to devices, systems and methodsfor reducing concentration of a chemical entity (for example, carbondioxide) in fluids such as blood in which an immobilized enzyme (forexample, carbonic anhydrase) is used to facilitate diffusion toward asurface or membrane.

The following information is provided to assist the reader to understandthe invention disclosed below and the environment in which it willtypically be used. The terms used herein are not intended to be limitedto any particular narrow interpretation unless clearly stated otherwisein this document. References set forth herein may facilitateunderstanding of the present invention or the background of the presentinvention. The disclosure of all references cited herein areincorporated by reference.

Artificial lungs are employed to oxygenate the blood and to remove CO₂.Hollow fiber membrane (HFM) based artificial lungs began to replacebubble oxygenators in the 1980s. In that regard, HFM-based artificiallungs exhibit improved gas exchange performance as compared to bubbleoxygenators. See Iwahashi H, Yuri K, Nose Y. 2004. Development of theoxygenator: past, present, and future. J Artif Organs 7:111-120. Nosedeveloped the first HFM type artificial lung in 1971. However, theperformance of early oxygenators was unacceptable as a result of fiberwetting and plasma leak problems. Nose Y, Malchesky P S. 1981; 3-14.Therapeutic membrane plasmapheresis. In: Therapeutic plasmapheresis. OdaT (ed) Stuttgart: F. K. Schattauer Subsequently, Kamo et al. developedcommercially available composite fibers which were constructed with atrue membrane layer between microporous walls. Kamo J, Uchida M, HiraiT, Yasuda H, Kanada K, Takemura T. 1990. A new multilayered compositehollow fiber membrane for artificial lung. Artificial Organs 14:369-372.Although, the composite fiber had excellent plasma wetting resistance,the permeance of the membrane was insufficient for intravenousoxygenation. Recent advances in membrane technology, however, haveenabled the development of noble membranes such as polyolefin-basedhollow fiber membrane that exhibit both good gas permeance and highplasma wetting resistance.

Currently available artificial lungs devices typically include bundlesof microporous hollow fiber membranes through which oxygen passes whileblood is perfused around the fibers. A review of artificial lungs andhollow fiber membrane technology is provided in Federspiel W J, HenchirK A. 2004. Lung, Artificial: Basic principles and current applications.Encyclo Biomat Biomed Eng 910-921, the disclosure of which isincorporated herein by reference. In general, oxygen is transferred fromthe lumen of the fibers into the blood; while CO₂ is transferred fromthe blood into the lumen of the fibers and is removed from the device.In the current artificial lung model, which is based on passivediffusion, the efficiency of CO₂ and O₂ gas exchange are limited by thefiber surface area to blood volume ratio. Gas exchange can be improvedby increasing this ratio at the cost of increasing the overall size ofthe artificial lung device. Additionally, CO₂ removal rates are limitedat lower blood flow rates.

Carbon dioxide is present in blood in three primary forms: CO₂(dissolved), bicarbonate (HCO₃ ⁻), or carbamate. As known in thechemical arts, CO₂ is interconvertible among these forms and the variousforms can be in equilibrium with each other as described by a CO₂dissociation curve. Most of the CO₂ in blood, however, exists in theform of HCO₃ ⁻ in plasma and in red blood cells. Colton C K. 1976.Fundamentals of gas transport in blood. In: Zapol W M and Qvist J,editor. Artificial lungs for acute respiratory failure. Washington D.C.:Hemisphere Publishing Corporation. p 3-43. In that regard, approximately94% of plasma CO₂ and 82% of red blood cell CO₂ is in the form of HCO₃⁻. The two species are interconvertible via the reaction:

The CO₂ generates via metabolic pathways in tissue and diffuses into redblood cells (RBCs), where it is hydrated into HCO₃ ⁻ and hydrogen ions(H⁺) by intracellular carbonic anhydrase (CA). The hydrogen ions formedare bound to hemoglobin while HCO₃ ⁻ is diffused into plasma. Jensen FB. 2004. Red blood cell pH, the Bohr effect, and other oxygenationlinked phenomena in blood O₂ and CO₂ transport. Acta Physiol Scand182:215-227. However, very little CO₂ is hydrated in plasma because of alack of CA in plasma. In lungs, the reaction is reversed. HCO₃ ⁻ isconverted into CO₂ via CA in red blood cells, and then exhaled. Some CAexists in lung tissue.

CA (EC 4.2.1.1; MW 30,000 Da) is a metalloenzyme with a single zincatom, which can effectively catalyze the reversible hydration anddehydration reaction of CO₂ (CO₂+H₂O

H⁺+HCO₃ ⁻). Cleland J L, Wang D I C. 1990. Refolding and aggregation ofbovine carbonic anhydrase B: Quasi-elastic light scattering analysis.Biochemistry 29:11072-11078; and Stemler. 1993. An assay for carbonicanhydrase activity and reactions that produce radiolabeled gases orsmall uncharged molecules. Anal Biochem 210:328-331. The enzyme enhancesboth hydration and dehydration rates over 10⁵-fold compared to reactionrates in the absence of CA, even though it is variable and depends onisoforms. Lindskog and Coleman, 1973 The catalytic mechanism of carbonicanhydrase. Proc Natl Acad Sci USA 70:2505-2508; Smith R G. 1988.Inorganic carbon transport in biological systems. Comp Biochem Physiol B90:639-654. Once again, CA is usually found within RBCs and lung tissue(alveolar epithelium).

CA has been used for CO₂ processing in a number of devices. For example,U.S. Pat. No. 6,946,288 discloses the use of CA to reduce CO₂ levels inair. In addition, U.S. Pat. No. 6,524,843 discloses a CA immobilizedbioreactor for the generation of CO₂. See also, U.S. Pat. No. 6,143,556and Published PCT International Patent Application Nos. WO 2006/089413and WO 2004-056455.

Moreover, a few studies have demonstrated that CA can improve CO₂removal in an oxygenator. Salley et al. evaluated CO₂ removal efficiencyusing an encapsulated CA in cellulose nitrate. Salley S O, Song J Y,Whittlesey G C, Klein M D. 1990. Immobilized carbonic anhydrase in amembrane lung for enhanced CO₂ removal. ASAIO Trans 36:M486-490. Salleyet al. immobilized CA containing microcapsules onto flat sheet typesilicone rubber membrane. They obtained about 60% enhanced CO₂ removalrate (2.58 ml/min for untreated membrane and 4.15 mL/min for CAimmobilized membrane). However, encapsulation resulted in an apparent80% loss of CA activity, which likely negates the improvement in CO₂exchange and enhanced storage stability of the encapsulated enzyme aswas reported in Salley S O, Song J Y, Whittlesey G C, Klein M D.Thermal, operational, and storage stability of immobilized carbonicanhydrase in membrane lungs. ASAIO J 1992; 38(3):M684-7. Mancini et al.performed CO₂ removal by employing extracorporeal CO₂ removal circuitswhich included a bubble oxygenator, a hollow fiber oxygenator to removeCO₂, and a dialyzer. Mancini II P, Whittlesey G C, Song J Y, Salley S O,Klein M D. 1990. CO₂ removal for ventilatory support: A comparison ofdialysis with and without carbonic anhydrase to a hollow fiber lung.ASAIO Trans 36:M675-678. Mancini et al. compared CO₂ removal performancewith and without free CA in the dialyzer. The CO₂ removal rates werefound to be 8.76 mL/min without CA and 12.18 mL/min with CA. However,the studies failed to achieve CO₂ removal performance exceeding normaloxygenator using the relatively complex CO₂ removal system describedtherein.

Although previous studies have demonstrated the presence of CA canimprove CO₂ gas exchange, the systems employed are not acceptable forpractical use in, for example, artificial lung and other respiratoryassist devices.

It remains desirable, for example, to develop improved artificial lungdevices, systems and methods. Preferably, such devices, systems andmethods are relatively simple in design and relatively efficient tomanufacture and to operate.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a device for removal of atleast a portion of carbon dioxide from an aqueous fluid, including: atleast one membrane through which carbon dioxide can pass to be removedfrom the fluid and immobilized carbonic anhydrase on or in the vicinityof a first surface of the membrane to be contacted with the fluid suchthat the immobilized carbonic anhydrase comes into contact with thefluid. The first surface exhibits carbonic anhydrase activity of atleast 20% of maximum theoretical activity of the first surface of themembrane based on monolayer surface coverage of carbonic anhydrase inthe case that the carbonic anhydrase is immobilize on the first surface.In several embodiments, the first surface exhibits carbonic anhydraseactivity of at least 40% of maximum theoretical activity of the fibersbased on monolayer surface coverage of carbonic anhydrase. Further, thefirst surface can exhibit carbonic anhydrase activity of at least 60% ofmaximum theoretical activity of the first surface of the membranes basedon monolayer surface coverage of carbonic anhydrase. Still further, thefirst surface can exhibit carbonic anhydrase activity of at least 80% ofmaximum theoretical activity of the first surface of the membrane basedon monolayer surface coverage of carbonic anhydrase. Even higheractivities are possible. Moreover, activities in excess of the maximumtheoretical activity of the first surface of the membrane based onmonolayer surface coverage of carbonic anhydrase in, for example,multilayered immobilization embodiments of the present invention. Inmultilayered embodiment, chemical chains including more than onecarbonic anhydrase group are immobilized on the first surface. Carbondioxide can, for example, be present in the fluid in the form ofbicarbonate ion.

The fluid can, for example, be blood and the membrane can, for example,be formed from a polymeric material. The fluid can also, for example, bean oxygenated perfluorocarbon. The carbonic anhydrase can, for example,be immobilized on the polymeric material via adsorption, covalentbonding, ionic bonding or chelation. In several embodiments, thecarbonic anhydrase is covalently attached to the polymeric material.

The polymeric material can, for example, be a microporous or permeablesuch that CO₂ can pass therethrough. In several embodiment, thepolymeric material is microporous and sufficiently hydrophobic so thatits pores remain gas filled after contacting blood or other aqueousfluids. The polymeric material can, for example, be an olefinicpolymeric material.

In several embodiments, the carbonic anhydrase is covalently attached tothe first surface of a microporous polymeric hollow fiber. The firstsurface can, for example, be an outer surface of the hollow fiber and aninterior lumen of the hollow fiber can be adapted to have oxygen (oranother carrier gas) flow therethrough. The hollow fiber can further beadapted to pass oxygen into the blood while carbon dioxide passes fromthe blood to the interior lumen of the hollow fiber. In severalembodiments, the carbonic anhydrase is covalently attached to apermeable, nonporous polymeric coating on an exterior surface of amicroporous polymeric hollow fiber. The device can include a pluralityof membranes formed by a plurality of hollow fibers.

The at least one membrane can also include or be formed from aCO₂-permeable silicone.

The device can further include free carbonic anhydrase to contact theblood.

The device can, for example, be a component of a total liquidventilation circuit adapted to be connected to lungs. The device canalso use peritoneal or gastric perfusion to provide respiratory support.Further, the device can also, for example, be a component of anartificial lung.

The polymeric material can be treated prior to immobilizing the carbonicanhydrase thereon to create reactive sites on the polymeric material.The polymeric material can, for example, be treated via radio frequencyplasma discharge to create reactive sites upon the polymeric material.The reactive sites can, for example, include at least one of a hydroxylgroup, an amine group or a carboxyl group.

In another aspect, the present invention provides a method for removalof at least a portion of carbon dioxide from an aqueous fluid,including: placing at least one membrane through which carbon dioxidecan pass to be removed from the fluid in contact with the fluid. Themembrane includes immobilized carbonic anhydrase on or in the vicinityof a first surface thereof such that the immobilized carbonic anhydrasecomes into contact with the fluid. The first surface exhibits carbonicanhydrase activity of at least 20% of maximum theoretical activity ofthe first surface of the membrane based on monolayer surface coverage ofcarbonic anhydrase in the case that the carbonic anhydrase isimmobilized on the first surface of the membrane.

In several embodiments, the fluid is blood or, for example, anoxygenated perfluorocarbon. In several embodiments, the carbonicanhydrase is immobilized on the first surface of the membrane.

As described above, the first surface can exhibit carbonic anhydraseactivity of at least 40% of maximum theoretical activity of the fibersbased on monolayer surface coverage of carbonic anhydrase, of at least60% of maximum theoretical activity of the fibers based on monolayersurface coverage of carbonic anhydrase, of at least 80% of maximumtheoretical activity of the fibers based on monolayer surface coverageof carbonic anhydrase, or even higher.

The carbonic anhydrase can be immobilized, for example, on a polymericmaterial via adsorption, covalent bonding, ionic bonding or chelation.In several embodiments, the carbonic anhydrase is covalently bonded tothe polymeric material. The carbonic anhydrase can, for example, becontacted with the polymeric material to react with reactive groups onthe polymeric material. The reactive groups on the polymeric materialcan, for example, be reactive with amine groups on the carbonicanhydrase.

In several embodiments, the polymeric material is treated via radiofrequency plasma discharge to create reactive sites upon the polymericmaterial. The reactive sites can, for example, include at least one of ahydroxyl group, an amine group or a carboxyl group.

In several embodiments, the polymeric material is contacted with acyanogen halide to react hydroxyl groups thereon with the cyanogenhalide. The carbonic anhydrase can be contacted with the polymericmaterial after contacting cyanogen halide with the polymeric material.The cyanogen halide can, for example, be cyanogen bromide.

Power levels during radio frequency plasma discharge and duration ofradio frequency plasma discharge can be maintained to preventsubstantially adverse effects upon the gas permeance of carbon dioxidethrough the polymeric material. Power levels during radio frequencyplasma discharge and duration of radio frequency plasma discharge canalso be maintained to prevent substantially adverse effects upon the gaspermeance of oxygen through the polymeric material.

As described above, polymeric materials used in the present inventioncan, for example, be microporous or permeable. The polymeric materialcan, for example, be formed into a microporous hollow fiber having aninterior lumen into which carbon dioxide can pass from the blood.

The method can further include causing oxygen to flow through theinterior lumen of the microporous hollow fiber, whereby the oxygen canpasses from the interior lumen into the blood.

In another aspect, the present invention provides a method ofmanufacturing a membrane for use in removal of carbon dioxide from anaqueous fluid including: immobilizing carbonic anhydrase on a firstsurface of a polymeric material via adsorption, covalent bonding, ionicbonding or chelation such that the first surface exhibits carbonicanhydrase activity of at least 20% of maximum theoretical activity ofthe first surface based on monolayer surface coverage of carbonicanhydrase.

In several embodiments, the carbonic anhydrase is covalently attached tothe first surface of the polymeric material, and the first surface ofthe polymeric material is treated prior to immobilizing the carbonicanhydrase thereon to create reactive sites on the polymeric material. Asdescribed above, the first surface of the polymeric material can betreated via radio frequency plasma discharge to create reactive sitesupon the first surface of the polymeric material. The created reactivesites can, for example, include at least one of a hydroxyl group, anamine group or a carboxyl group.

In another aspect, the present invention provides a device for removalof at least a portion of carbon dioxide from an aqueous fluid,including: at least one membrane through which carbon dioxide can passto be removed from the fluid and immobilized carbonic anhydrase in thevicinity of a first surface of the membrane in contact with the fluidsuch that the immobilized carbonic anhydrase comes into contact with thefluid. The carbonic anhydrase can, for example, be immobilized on amaterial positioned in the vicinity of the at least one or surface. Thematerial can, for example, be a metal. In several embodiments, thematerial is gold. The gold can, for example, be in the form ofnanoparticles.

In still another aspect, the present invention provides a device forremoval of at least a portion of chemical entity from a fluid,including: at least one membrane through which the chemical entity canpass as a gas to be removed from the fluid and immobilized enzyme on orin the vicinity of a first surface of the membrane to be contacted withthe fluid such that the immobilized enzyme comes into contact with thefluid. The first surface can exhibit enzyme activity of at least 20% ofmaximum theoretical activity of the first surface based on monolayersurface coverage of enzyme in the case that the enzyme is immobilize onthe first surface. The chemical entity is inter-convertible within thefluid to at least one other chemical entity. The enzyme is functional tocatalyze a reaction of the other chemical entity to the chemical entity(to be removed).

In several embodiments, the devices, systems and methods of the presentinvention can easily be incorporated or retrofitted into existingartificial lung devices and/or respiratory assist devices. In the caseof artificial lung devices and/or respiratory assist devices including amembrane or membranes such as hollow fiber membranes, for example, noadditional components are required as compared to existing devices. Themanufacture requires only the additional process of immobilizing CA (forexample, via covalent bonding) on, for example, the outer surfaces ofmembranes and/or hollow fibers. The operation of the artificial lungdevices remains the same, while CO₂ removal is significantly improved.The significant improvement in the rate of CO₂ removal provided by thedevices of the present invention can result in corresponding decreasesin the total membrane surface area and a reduction in overall devicesize required in systems for removal of CO₂. This can, for example, be asignificant advantage in the development of implantable artificiallungs.

The present invention, along with the attributes and attendantadvantages thereof, will best be appreciated and understood in view ofthe following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a comparison of the operation of a standard hollowfiber membrane with a membrane of the present invention in whichdiffusion is facilitated by carbonic anhydrase immobilized on or in thevicinity of the membrane surface in contact with the blood plasma. Thefacilitation occurs because the immobilized enzyme generates diffusionof bicarbonate ion (an alternative form of CO₂), which is a substrate ofthe enzyme, towards the membrane. At the membrane the bicarbonate ion isconverted to CO₂ which then diffuses into and across the membrane.

FIG. 1B illustrates an RFGD apparatus, a mechanism for immobilization ofcarbonic anhydride on a polymeric material.

FIG. 2A illustrates a study of the effect of RFGD treatment power andtreatment duration on gas permeance of polymethyl-pentene (PMP) hollowfiber membranes wherein CO₂ was used as a gas source and thedesignation * represent that p<0.05 between the sample and the control(unmodified fiber).

FIG. 2B illustrates SEM analysis of RFGD modified PMP hollow fibermembranes wherein the hollow fibers were exposed to water plasma over arange of powers and exposure times under a 400 mTorr vacuum, and theimages were taken at 100,000 times magnification using an acceleratingvoltage of 10 kV, wherein slide (A) sets forth a micrograph ofunmodified HFM, slide (B) sets forth a micrograph of HFM modified usinga power and duration of 50 W and 60 s, respectively, slide (C) setsforth a micrograph of HFM modified using a power and duration of 100 Wand 90 s, respectively, and slide (D) sets forth a micrograph of HFMmodified using a power and duration of 100 W and 180 s, respectively.

FIG. 3 illustrates a study of the effect of RFGD plasma depositionconditions on enzyme immobilization efficiency wherein the dashed lineindicates the theoretical enzyme activity at monolayer CA coverage.

FIG. 4 illustrates a study of enzyme immobilization efficiency withvarying CA concentration in the coupling solution wherein various levelsof enzyme activity of the fibers as a function of CA loading wereachieved, and the dashed line indicates the theoretical enzyme activityat monolayer coverage of CA on outer surface of the fibers.

FIG. 5 illustrates an embodiment of a mini-lung and CO₂ removalmeasurement system of the present invention as used in several studiesof the present invention.

FIG. 6 illustrates an evaluation of CO₂ removal by adding variousactivities of free CA wherein the surface area of HFM was 71.6 cm² andwherein error bars denote the standard deviation (n=2).

FIG. 7 illustrates an evaluation of CO₂ removal by immobilizing variousenzyme activity levels of CA on HFM wherein the surface area of HFM was74 cm² and wherein error bars denote the standard deviation (n=2) (CAtest mini-lung module and others testing conditions were the same in thestudies of FIGS. 6 and 7).

FIG. 8 illustrates a comparison of mass transfer coefficients forvarious fibers wherein the liquid flow rate and sweep gas flow rate inthe mini-lung module were set at 10 mL/min and 30 mL/min, respectively.

FIG. 9A illustrates a schematic representation of layer-by-layer CAassembly using a glutaraldehyde chemistry in which glutaraldehyde(having two aldehyde end groups) was used to covalently cross linkbetween amine containing molecules such as CA and PLL in which CA wasfirst immobilized.

FIG. 9B illustrates a schematic of layer-by-layer CA assembly using aglutaraldehyde chemistry in which glutaraldehyde was used to covalentlycross link between amine containing molecules such as CA and PLL inwhich PLL was first immobilized.

FIG. 9C illustrates graphically increases in esterase activity resultingfrom increasing number of CA layers deposited on HFM surface.

FIG. 10A illustrates quantification of activated platelet by flowcytometry using annexin V as a marker.

FIG. 10B illustrates a comparison of activated platelet adhesion by SEMin which slides A and B illustrate unmodified hollow fiber membranes andslides C and D illustrates CA-immobilized hollow fiber membranes at ×200and ×1000 magnification, respectively.

FIG. 11A illustrates CO₂ removal from bovine blood using unmodified andCA-immobilized hollow fiber membranes in the mini-lung (passive mixing)system similar to that of FIG. 5.

FIG. 11B illustrates rate constants for unmodified hollow fibermembranes and CA-immobilized hollow fiber membranes in removal of CO₂from bovine blood, wherein the rate constants shown are linearlyproportional to mass transfer coefficients for removal from blood.

FIG. 12A illustrates a partially cutaway view (wherein a portion of thehousing is cutaway to show the HFM therein) of an embodiment of acommercially available artificial lung into which HFM includingimmobilized CA of the present invention can be incorporated.

FIG. 12B illustrates an embodiment of a paracorporeal respiratory assistlung (PRAL) of the present invention in an assembled state.

FIG. 12C illustrates the paracorporeal respiratory assist lung of FIG.9B in a disassembled state.

FIG. 12D illustrates an embodiment of a intravascular respiratory assistcatheter of the present invention.

FIG. 13A illustrates a cross-sectional representation of a microporoushollow fiber without a nonporous, permeable coating.

FIG. 13B illustrates a cross-sectional representation of a microporoushollow fiber with a nonporous, permeable coating.

FIG. 13C illustrates a scanning electron micrograph of a microporoushollow fiber without a nonporous, permeable coating.

FIG. 13D illustrates a scanning electron micrograph of a microporoushollow fiber with a nonporous, permeable coating.

FIG. 14 illustrates a scanning electron micrograph of an HFM fabric inwhich woven support threads are used to connect the hollow fibers.

FIG. 15 illustrates an idealized representation of a hollow fiber uponwhich carbonic anhydrase has been immobilized upon the outer surface ofthe hollow fiber, and wherein oxygen passes from the lumen of the hollowfiber to blood on the exterior of the fiber and carbon dioxide passesfrom the blood on the exterior of the hollow fiber to the lumen of thehollow fiber.

FIG. 16 illustrates scanning electron micrographs of microporous hollowfiber membranes suitable for use in artificial lungs wherein the wallsof the fibers (right) contain submicron pores through which respiratorygases diffuse.

DETAILED DESCRIPTION OF THE INVENTION

As used herein and in the appended claims, the singular forms “a,” “an”,and “the” include plural references unless the content clearly dictatesotherwise. Thus, for example, (unless clearly indicated otherwise)reference to “a chemical entity”, to “a membrane” or to “a hollow fiber”includes a plurality of such chemical entities, membranes or hollowfibers and equivalents thereof known to those skilled in the art, and soforth, and reference to “the chemical entity”, to “the membrane” or to“the hollow fiber” is a reference to one or more such chemical entities,membranes or hollow fibers and equivalents thereof known to thoseskilled in the art, and so forth.

As set forth above, the present invention provides devices, systems andmethods for reducing the concentration of a chemical entity in fluids(for example, liquids, gases and combinations thereof). In severalrepresentative embodiments set forth below, the present invention isillustrated by setting forth devices, systems and methods for reducingthe concentration of carbon dioxide in fluids such as blood in whichimmobilized carbonic anhydrase is used to facilitate diffusion toward amembrane including the immobilized enzyme. One skilled in the art,appreciates that the concepts of the present invention are applicable toenzymes generally as used in devices, systems and method to remove atleast a portion of a chemical entity from a fluid. Moreover, the methodsof immobilizing enzymes set forth in the present invention areapplicable generally to any enzyme.

It was hypothesized that the rate of CO₂ removal could be considerablyaccelerated through adding free CA and/or immobilizing CA in thevicinity of or upon at least the outer surface of a gas permeablepolymeric material such as the HFMs of an artificial lung and otherdevices. In that regard, and without limitation to any mechanism, it washypothesized that the enzyme would catalyze the conversion ofbicarbonate in the blood to CO₂, which can diffuse into the lumen of thefibers and be excreted, thereby facilitating active gas exchange.Increasing the efficiency of CO₂ removal is important in the use of HFMsin respiratory assist devices because the natural concentration gradientfor CO₂ diffusion is much smaller than that for O₂ addition, resultingin a blood flow-dependent limitation to exchange. Furthermore, in manypatients with respiratory failure the need for CO₂ removal is moreimportant clinically, as oxygenation can be provided by nasal cannula orby low tidal volume, lung-protective ventilation

Human tissues face the same diffusional challenges, and blood cells andthe surface of the lung are coated with carbonic anhydrase, whichaccelerates diffusion across the small gradient. As discussed above, CAis present in red blood cells and on the endothelial surfaces of lungcapillaries to aid in the carriage and exchange of CO₂. By catalyzingthe reversible hydration of CO₂ into carbonic acid, which then rapidlydissociates into bicarbonate ion, CA substantially increases the CO₂carrying capacity of blood, with over 90% of the CO₂ carried in bloodbeing in the form of bicarbonate.

In several embodiments of the present invention, “bioactive” membranesystems such as bioactive HFM systems were developed to, for example,improve respiratory assist devices for CO₂ removal in lung failurepatients. Employing a biomimetic approach, CA was immobilized on or inthe vicinity of the surface of, for example, conventional HFMs, enabling“facilitated diffusion” of CO₂ as bicarbonate towards the HFM andenhancing the removal rate of CO₂. FIG. 1A illustrates a comparison ofthe operation of a standard hollow fiber membrane with a membrane of thepresent invention in which diffusion is facilitated by CA immobilized on(or in the vicinity of) the membrane surface in contact with the plasma.

With reference to FIG. 1A, in the present invention, removal of aspecific chemical entity (for example, CO₂) from a fluid (for example,blood) is facilitated via enzyme-catalyzed reaction. The chemical entityto be removed exists within the fluid in one or more different chemicalforms or as one or more other chemical entities (for example, thechemical entity CO₂ can exist as HCO₃, H₂CO₃ etc). The different formsof the chemical entities should be inter-convertible within the fluidand may or may not be in equilibrium with each other. At least one ofthe other forms of the specific chemical entity to be removed is asubstrate for an enzyme that can convert the other form(s) to thespecific chemical entity to be removed. In general, the chemical entityto be removed will have a non-negligible partial pressure and can existas a gas under the operating conditions to pass through a gas porous orgas permeable membrane. The other form(s) of the chemical entity aretypically solutes that cannot exist as a gas under the operatingconditions and cannot pass through the membrane. The immobilized enzymefacilitates diffusion of the specific chemical entity through themembrane by generating diffusion of least one of the other forms of theentity to the surface of the membrane. At the membrane surface theenzyme converts at least one of those other forms to the specific form(the chemical entity) to be removed.

The studies of the present invention illustrated that CA maintainssubstantial activity upon immobilization (for example, via covalentbonding to a polymeric material). Immobilization may, for example,provide increased activity as compared to encapsulation (in which accessto the CA can be hindered). Immobilization can, for example, be effectedvia adsorption, chemical bonding or chelation. Preferably, relativelystrong attachments are used in immobilization such as covalent bonding,ionic bonding or chelation.

In several representative studies of the present invention, combinedradio frequency glow discharge (RFGD) and cyanogen bromide (CNBr)activation chemistry was used to immobilize CA on an outer surface ofthe HFM via covalent bonding under experimentally determined conditions.In that regard, conditions were developed that, for example, did notsubstantially negatively affect the structural integrity of the HFM. Tocharacterize the surface of the HFM before and after the CAmodification, SEM analysis and gas permeance measurements were employed.The impact of enzyme activity levels of free and immobilized CA on CO₂removal were also studied using an HFM mini-lung module. CO₂ removalstudies were performed using bicarbonate solution using both free andimmobilized CA in the mini-lung module as a small scale artificial lung.Once again, bicarbonate in the plasma is the primary form of CO₂ foundin blood.

As illustrated, for example, in FIG. 1B, CA was immobilized onto thefiber surface via initially introducing hydroxyl groups onto the fibersusing radio frequency glow discharge (RFGD). The hydroxyl groups weresubsequently converted to an imidocarbonate intermediate by the additionof cyanogen bromide. CNBr and other cyanogen halide immobilizationchemistry is reviewed, for example, in Axén, R. and Ernback, S, (1971),Chemical Fixation of Enzymes to Cyanogen Halide Activated PolysaccharideCarriers, Eur. J. Biochem, 18:351-360. Incubation with a solution of CAenabled the reaction between the cyanogen bromide-activated fibers andsurface hydroxyls to convert the surface hydroxyls to cyanate esters andcyclic imidocarbonate to which CA can subsequently be reacted withforming covalent isourea and N-substituted imidocarbonate linkage.Parameters that impact the enzyme coupling reaction include theconcentration of CA relative to the number of activated fibers, pH ofthe reaction buffer, and reaction time,

The immobilization of CA (and/or other enzymes) to the HFMs and/or otherpolymeric materials is not limited to the above-described chemistry.Alternative routes of immobilization include introducing amine groupsonto the fiber surface via RFGD and subsequently conjugating CA to theamines using glutaraldehyde as a crosslinker. CA may also be immobilizedvia the use of photocrosslinkers such as N-succinimidyl 4-benzoylbenzoicacid through a two step conjugation process. In the first step, the HFMsare activated by modification with one end of the crosslinker.Benzoylbenzoic acid, for example, can be reacted to the poly(methylpentene) backbone by irradiation with UV (350-380 nm) light. Uponactivation of the fibers, CA can then be conjugated to the other end ofthe crosslinker, which should contain a protein reactive functionalgroup. The N-succinimidyl group is commonly employed on protein reactivecrosslinkers because it reacts rapidly with amines in the protein underambient temperature and physiological pH. Other routes for covalentlyimmobilizing CA onto a polymer (for example, a hollow fiber membrane)surface include biotinylation of CA and subsequent binding tostreptavidin-modified polymer and incorporation of CA into a polymercoating that can be wrapped around the fiber bundles without affectinggas exchange. In the synthesis of CA-immobilized coatings, functionalgroups on the enzyme's surface enable multipoint covalent incorporationof the enzyme directly into the polymer matrix, thus preventing leachingof the enzyme. As described above, CA may also be attached to the HFMs,although non-covalently, through, for example, physical adsorption,ionic interaction or chelation.

Chemical attachment techniques relevant to attachment of enzymes to, forexample, polymeric materials are discussed generally, for example, inHermanson, G. T. (1996). Bioconjugate techniques. San Diego, AcademicPress and Hermanson, G. T., A. K. Mallia, et al. (1992). Immobilizedaffinity ligand techniques. San Diego, Academic Press.

Additionally, CA and/or other enzymes can be incorporated into thepolymeric matrix or backbone of a polymer. For example, active CA and/orother enzymes can be incorporated into polyurethanes as described, forexample, in U.S. Pat. No. 5,482,996.

Non-polymeric materials such as metals may also be employed as suitablesupport structures/surfaces for enzyme immobilization (such as inCA-immobilization in, for example, an artificial lung application).Amines and thiols, which are present on the surface of enzymes, reactrapidly with a variety of metals including gold and silver and thus, inmany cases, eliminate the need to pre-activate the metallic substrate.Bifunctional crosslinking agents which form self-assembling monolayerson metal surfaces are also used to covalently attach enzymes such as inthe construction of amperometric bionsensors (Pariente F, La Rosa C,Galan F, Hernández L, Lorenzo E. Enzyme support systems for biosensorapplications based on gold-coated nylon meshes. Biosens Bioelectron1996; 11:1115-1125). CA can potentially be immobilized to nano or microsized metallic particles whose primary role in, for example, anartificial lung is to disrupt diffusional boundary layers on the HFMs.

In the representative studies of the present invention, RFGD treatmentwas used as the first and primary step in introducing functional groupson the fibers to enable surface attachment of CA. However, severalreports have shown RFGD to alter surface properties such as surfaceroughness and to cause surface damage. See, for example, Drnovská H,Lap{hacek over (e)}ik Jr L, Bur{hacek over (s)}ikova V, Zemek J,Barros-Timmons A M. 2003. Surface properties of polyethylene afterlow-temperature plasma treatment. Colloid Polym Sci 281:1025-1033 andMalpass C A, Millsap K W, Sidhu H, Gower L B. 2002. Immobilization of anoxalate-degrading enzyme on silicone elastomer. J Biomed Mater Res63:822-829. Preferably, the RFGD modification for surface activation ofthe HFM does not substantially affect or alter the original gaspermeance characteristics, which could, for example, indicate surfacedefects that would allow blood plasma into membrane pores and causedecreases in gas permeance. Consequently, the physical integrity of theHFMs after RFGD plasma treatment was investigated by measuring gaspermeance and by scanning electron microscope (SEM) analysis.

Gas permeance (Km) is a measure of the speed with which a gas candiffuse across a membrane. The gas permeance is the amount of gas flowper unit area of membrane that would arise from a unit difference in gaspartial pressure across the membrane. For CO₂, the gas permeance isgiven by

$K_{m} = \frac{{\overset{.}{V}}_{{CO}\; 2}}{A\;\Delta\; P_{{CO}\; 2}}$

where {dot over (V)}_(CO2) is the CO₂ flowrate across the membrane, A isthe membrane area, and ΔP_(CO2) is the partial pressure difference ofCO₂ across the membrane.

For artificial lung applications it is important that Km be large enoughthat CO₂ removal is not limited by diffusion across the membrane. Forthis reason microporous fibers were developed to replace the nonporoussilicone membranes used in earlier artificial lung and blood oxygenatordesigns. Microporous fibers are fabricated using a variety of techniquesto create submicron sized voids or pores in the polymer during membranefabrication. For example, the membrane may be stretched during extrusionof the polymer as it leaves the dye or spinneret. The stretching causesdefects (openings, pores, voids etc) to form in the polymer melt as itis crystallizing. These pores remain permanent once the fiber is cooledand conditioned at normal temperatures. Typically the porosity of thesemembranes range from 30-50%.

A microporous hydrophobic membrane has a much greater Km than anonporous polymer membrane. If the pores remain gas filled, amicroporous membrane allows gas diffusion through a gas phase ratherthan a solid polymer phase, as would occur in nonporous polymermembranes. The diffusion coefficients in gases are substantially higherthan those in solid nonporous polymers. For this reason, the Km for amicroporous membrane with a thickness of 25 microns (a typical membranewall thickness in hollow fibers) varies from about 10⁻² mlCO₂/cm²/s/cmHg to 10⁻¹ ml CO₂/cm²/s/cmHg. In contrast, a nonporoussilicone-type membrane (siloxane polymers are some of the most gaspermeable) of the same thickness would have a Km of about 10⁻⁶ mlCO₂/cm²/s/cmHg to 10⁻⁵ ml CO₂/cm²/s/cmHg. A permeance this small limitsthe CO₂ removal rate of the membrane and the device in which themembrane is incorporated.

In artificial lung and blood oxygenator applications microporousmembranes can “wet-out”. Plasma seeps into the pores and dramaticallydecreases the Km of the membrane. For this reason, microporous membraneshave been developed that have thin regions or layers of nonporouspolymer on the blood-contacting side of the membrane. The membranes areeither of asymmetric or composite design as discussed further below.These nonporous polymer layers block plasma infiltration but also reducethe permeance of the membrane. It is important that the nonporouspolymer regions be thin compared to the thickness of the membrane.Typically, if siloxane nonporous polymers are coated on microporousmembranes the coating is less than approximately 1 micron.

Preferably, the membrane Km for CO₂ in artificial lungs and bloodoxygenators should be greater than 10⁻² ml CO₂/cm²/s/cmHg. Suchmembranes will have a negligible effect on CO₂ removal from the device.A permeance this high is difficult to achieve if an asymmetric orcomposite membrane is needed to prevent plasma wetting by blood.Practically then, a permeance greater than 10⁻³ ml CO₂/cm²/s/cmHg isdesired and achievable for asymmetric and composite fibers. Suchmembranes would affect CO₂ removal by less than 5-10% under nominaloperating conditions.

To study the effect of RFGD plasma treatment on gas permeance, a rangeof RFGD modification conditions for coupling of CA onto the outersurface of HFMs of the artificial lungs was studied. As illustrated inFIG. 2A, the gas permeance of the HFM was found to increasesignificantly with increasing exposure time and discharge power of theRFGD treatment. The gas permeance is a measure of the integrity of thesurface of the PMP fiber, which was an asymmetric fiber in which theporosity goes to zero at the fiber surface. If permeance increasessignificantly this indicates that the fiber surface has been compromisedby the RFGD treatment and would likely increase the potential for bloodplasma wetting.

Samples treated at 50 W for 60 s, 100 W for 90 s, and 100 W for 180 sexhibited increased gas permeances of about 1.85-, 4.30-, and 7.61-fold(using CO₂ as a gas source), respectively, compared to unmodifiedmembrane (control). These RFGD treatment conditions inducedstatistically significant differences in gas permeance compared to thecontrol (p<0.02 between the sample and the control). In contrast, nochange of gas permeance was observed in the sample treated at 25 W powerfor 30 s in comparison with the unmodified fiber regardless of gassource (p>0.06). For HFM exhibiting desirable gas permeances, increasesin gas permeance are preferably minimized. In general, it is preferredthat gas permeances increase by less then 25%, and, more preferably, byless than 10%. Even more preferably, gas permeance changes by less than5%. In general, power can be preferably maintained at 50 W or less inthe RGFD used in the studies of the present invention. As power isincreased, however, duration of treatment should be decreased. Forexample, at 50 W, a treatment time of 30 s can result in an increase ingas permeance of about 50%.

These results indicated RFGD treatment damaged the structural integrityof the fibers when sufficient power was used for a sufficient period oftime. However, among the RFGD treatment conditions tested, the 25 W and30 s treatment did not cause physical damage on the outer surface of thefibers as indicated by gas permeance testing of FIG. 2A and SEM analysisset forth in FIG. 2B (which is discussed further below). As alsodiscussed further below, the 25 W and 30 s RFGD treatment conditionsprovided for a relatively good enzyme immobilization efficiency ofapproximately 80% surface coverage using 1 mg/mL of CA in a couplingsolution.

To verify the effect of plasma treatment on the structural integrity ofthe fibers, various conditions of RFGD treated PMP HFMs (unmodified, 50W for 60 s, 100 for 90 s, and 100 W for 180 s) were analyzed using SEManalysis as illustrated in FIG. 2B. SEM images of the RFGD modifiedfibers showed visual changes in the surface roughness of the samples.The RFGD treatment caused defects on the outer surface of the fibers inproportion to time and power. The sample treated at 100 W for 180 sexhibited significant surface damage, including cracking on the fiber(see FIG. 2B, slide D). The sample treated at 50 W for 60 s (FIG. 2B,slide B), however, exhibited a similar outer surface to a unmodifiedmembrane (FIG. 2B, slide A).

The enzyme activity on the HFMs as a function of CA loading for variousconditions of RFGD treatment was evaluated as illustrated in FIG. 3. Theimmobilization efficiency of the sample treated at 25 W for 30 s, basedon the enzyme activity assay, was lower than that of the sample treatedat 100 W for 90 s. However, no statistical difference was observedbetween the two groups (p=0.22). Up to 88% functional CA coverage on thefiber surfaces was achieved using samples treated with RFGD at 25 W for30 s. For the purposes of the present studies, it was concluded thattreatment conditions of 25 W and 30 s were sufficiently optimized withregard to prevention of fiber damage and CA loading.

The effect of CA concentration (5, 50, 200, and 1000 μg/mL) in acoupling solution on enzyme immobilization efficiency was also studied.Results demonstrated that enzyme activity (U) of the fibers as afunction of CA loading was proportional to CA concentration. Fibers with0.09, 0.13, 0.17, and 0.27 U were obtained as illustrated in FIG. 4 forCA concentrations of 5, 50, 200, and 1000 μg/mL, respectively.Additionally, the functional CA coverage on the fibers was determined bycalculating the percentage of the actual enzyme activity on the fibersmeasured over the theoretical enzyme activity. The enzyme activitylevels set forth above represent 26, 38, 50, and 79% of maximumtheoretical activity of the fibers based on monolayer surface coverageof CA (2710 U of native enzyme used) on the outer surface of fibers. Incalculating, maximum theoretical activity based on a monolayer surfacecoverage, it was assumed that a single molecule of CA covers 44.5 nm²(6.67×6.67 nm). Saito et al. Acta Cryst. 2004. D60. 792-795. Thus, CAcould be immobilized as much as 112 ng/cm² in case of theoreticalmonolayer coverage. Thus, one can determine the amounts and % of CAimmobilized through CA activity assay (esterase activity measurement).As, the surface area of the HFM used in the studies was 50 cm², theenzyme activity levels set forth above correspond to specific enzymeactivities of 0.0018 U/cm², 0.0026 U/cm², 0.0034 U/cm² and 0.0054 U/cm².In general, specific enzyme activities of at least 1.5×10⁻⁴ U/cm² arereadily achievable in the present invention. In a number of preferredembodiments, specific activity of membranes (for example, HFM) of thepresent invention is at least 1.5×10⁻³ U/cm², or even at least 3.0×10⁻³U/cm², or even at least 5×10⁻³ U/cm², or even at least 1.0×10⁻² U/cm².

The relationship between gas permeance and CA loading on the HFM wasinvestigated. For the gas permeance tests results set forth in Table 1,O₂ as well as CO₂ were used as the gas source. In these tests, it wasassumed that the gas permeance may be affected by immobilized CA whenCO₂ is used as a gas source since CO₂ is a substrate of the CAimmobilized on the outer surface of the HFM. Thus, O₂ was additionallychosen as an “inert” gas source, although CO₂ is a target test gas ingas exchange experiments. No relationship was observed between levels ofcovalently immobilized CA and gas permeance, regardless of gas source.This indicates that the CA was attached in a manner that does not impedediffusion of gas into the microporous fiber. Without limitation to anymechanism, this observations may be a result of the binding pattern ofCA on the membrane surfaces. In that regard, the enzymes may be anchoredon the outer surface of the fibers with micro, spot-like contact as evenone covalent bond can be effective to immobilize ligand molecules. Inother words, the contact area of the immobilized CA may be negligiblecompared to the total outer surface area of the fibers. This observationindicates that a higher level of CA immobilization is not a limitingfactor in terms of the rate of gas exchange and that it can bebeneficial to immobilize as much CA per unit surface area as possible toaccelerate the rate of CO₂ removal from blood.

Table 1 sets forth data on the relationship between enzyme activitylevels of the fibers obtained above (0, 0.09, 0.13, 0.17, and 0.27 U)and their gas permeance. Once again, the amount of CA immobilized ontothe fiber surface had no effect on the gas permeance of the HFMs.Comparison of results for control samples (non-CA immobilized fiber thatwere, however, RFGD and CNBr activated) with the other samples showed nostatistically significant difference regardless of gas source (CO₂ orO₂). The p values through Student t-test ranged between approximately0.18-0.90 for CO₂ and between approximately 0.06-0.94 for O₂. Thecontrol had higher gas permeance than that of the intact (non-activated)fiber, but the difference was negligible (data for intact fiber are notshown). Statistical comparisons were performed using a Student's t-testassuming two-tailed data distribution and equal sample variance. Theresults of the gas permeance evaluation were analyzed using the aboveassumptions. In general, statistical significance was defined as p<0.05in the studies of the present invention.

TABLE 1 Relationship between CA activity levels of the fibers as afunction of CA loading and their gas permeance Enzyme activity Testvariables (U) K (mL/s/cm²/cmHg) K (%) p value (1) For CO₂ as a gassource 0 μg/mL CA 0 1.64E−03 ± 1.83E−04 100 — 5 μg/mL CA 0.09 1.82E−03 ±7.71E−05 111 *0.18 50 μg/mL CA 0.13 1.62E−03 ± 7.07E−05 99 **0.90 200μg/mL CA 0.17 1.78E−03 ± 9.68E−04 109 ^(†)0.30 1000 μg/mL CA 0.271.79E−03 ± 6.75E−05 109 ^(‡)0.24 (2) For O₂ as a gas source 0 μg/mL CA 01.44E−03 ± 2.16E−04 100 — 5 μg/mL CA 0.09 1.74E−03 ± 6.64E−05 120 *0.0850 μg/mL CA 0.13 1.42E−03 ± 8.83E−05 99 **0.94 200 μg/mL CA 0.171.78E−03 ± 1.07E−04 124 ^(†)0.07 1000 μg/mL CA 0.27 1.79E−03 ± 7.14E−05124 ^(‡)0.06 **0 versus 5 μg/mL CA ***0 versus 50 μg/mL CA ^(†)0 versus200 μg/mL CA ^(‡)0 versus 1000 μg/mL CA

To evaluate the impact of CA on CO₂ removal performance, a CO₂ detectionsystem was constructed including fabrication of an HFM mini-lung module10 as illustrated in FIG. 5. CO₂ removal was first evaluated by addingfree CA in the min-lung module 10 of FIG. 5 wherein the module containedunmodified fibers 20. The purpose of these experiments was to collectreference CO₂ removal data for various enzyme activity levels and tocompare the removal efficiency with that obtained in experimentsincluding fibers with immobilized CA. CO₂ removal was evaluated byadding various activities of free CA (0.25, 0.5, and 1 U in 25 mM NaHCO₃solution using the mini-lung device containing unmodified HFMs. Asillustrated in FIG. 6, the rate of CO₂ removal increased 1.71-, 2.09-,and 2.76-fold, respectively, compared to that of the control (noaddition of CA). No physical adsorption of CA on the HFM was observedduring each circulating procedure of free CA (data not shown). Theresults demonstrated that the CO₂ removal rate is a function of enzymeactivity.

CO₂ removal for various enzyme activity levels of CA immobilized HFM wascompared under a constant liquid flow rate (10 mL/min) and sweep gasflow rate (30 mL/min). As illustrated in FIG. 7, CO₂ removal in themini-lung containing fibers with 0.2, 0.25, and 0.3 U of immobilized CAactivity were improved by approximately 48%, 64%, and 75%, respectively.No enzyme desorption from the fibers was observed in these experiments(data not shown).

FIG. 8 illustrates a comparison of mass transfer coefficient for variousconditions of fibers. HFM that underwent RFGD and CNBr activationwithout enzyme modification did not exhibit changes in mass transfercoefficient as compared to unmodified HFM. The enzyme immobilized fibersshowed similar mass transfer coefficients to an unmodified HFM modulecontaining the same enzyme activity level of free CA (0.25 U).

The gas exchange efficiency for CO₂ removal using CA immobilized fiberswas slightly lower than that of free CA, but the results were quitesimilar (see FIGS. 7 and 8). The efficiency of CO₂ exchange can befurther improved by increasing the amount of immobilized enzyme activityon the fibers. A 3-dimensional immobilization of CA on the HFM can, forexample, be used to significantly improve enzyme immobilizationefficiency beyond monolayer attachment.

In that regard, the immobilization of CA onto cyanogensbromide-activated HFMs yields a single layer of enzyme on the fibersurface. As discussed above, one strategy to increase the amount of CAand thus the enzymatic activity on the fibers is to immobilize multiplelayers of enzyme in a 3-dimensional immobilization. Bifunctionalcrosslinking agents such as glutaraldehyde, which reacts with primaryamines, and EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide), whichreacts with carboxylic acids, may be employed to conjugate additional CAto the surface-immobilized CA. Successive crosslinking steps willultimately result in the formation of a multilayer CA shell withactivity enhancements potentially several fold greater than achievedwith the current model.

In several studies, CA loading was increased by immobilizing multipleenzyme layers on the surface of HFM as discussed above. FIG. 9A setsforth a schematic representation of the layer-by-layer assembly ofsurface immobilized CA in which glutaraldehyde is used as across-linking agent. CA was initially immobilized on the HFM using thegeneral procedure previously outlined. The CA-immobilized HFM weresubsequently immersed in a 0.5% (v/v) glutaraldehyde solution for 30 minwith mild shaking at room temperature, resulting in the modification offree primary amines on the enzyme surface. Any non-covalently attachedglutaraldehyde was removed from the HFM surface by thorough rinsing ofthe HFM with deionized water. The glutaraldehyde-treated HFM were thenreacted with CA at a concentration of 1 mg/mL in buffer (50 mMphosphate, pH 7.5) for 1 hr. In this way, primary amines in the enzyme,such as the ε-amine on the side chain of lysine residues or theN-terminal α-amine, will react with the remaining free aldehydefunctional group of glutaraldehyde, forming a second layer of CA. Thetwo step process of glutaraldehyde treatment and subsequent reactionwith additional enzyme was repeated to assemble up to 5 layers of enzymeon the surface of HFM (see FIG. 9A). Increases of 71% and 112% inesterase activity were measured for HFM with 3 and 5 layers ofimmobilized CA respectively relative to HFM modified with monolayer CAcoverage (FIG. 9C).

As clear to one skilled in the art, use of multiple layers ofimmobilized CA can result in enzyme activity greater than the maximumtheoretical activity based upon monolayer surface coverage. In general,activity resulting from multiple layers is additive as compared to acorresponding single layer.

The loading of CA onto surface of HFMs was also be increased byinitially modifying the surface of HFM with poly-L-lysine (PLL; M_(w)70-150 kDa), which in effect introduces additional reactive sites for CAimmobilization. Using this approach, PLL was initially immobilized ontoplasma-activated HFM that had been reacted with CNBr. The PLL-modifiedHFM were reacted with glutaraldehyde and subsequently with CA asdescribed above. Successive treatments of glutaraldehyde and CA wereapplied to add additional layers of immobilized CA (see FIG. 9B). CAloading was improved by 159% by constructing 3 layers of immobilized CAonto HFM compared to CA-immobilized HFM prepared with the standardmethod (see FIG. 9C).

In the use of HFM including immobilized CA in artificial lung systemsfor clinical use, it can also be beneficial to protect the enzyme fromproteases in blood. Such protection can, for example, be achieved bymodification of the enzyme such as a PEGylation to maintain enzymeactivity. In that regard, covalent binding of polyethylene glycol (PEG)to enzymes is known to improve stability.

Proteolytic degradation of the immobilized CA and non-specific bindingbetween CA and plasma proteins may reduce the catalytically activehalf-life of the CA-modified HFMs upon exposure to circulating blood. Ingeneral, many therapeutic enzymes are stable for times on the order ofonly minutes to hours as a result of physiological clearance mechanisms.One approach to prolonging the bioactivity of the fibers is tocovalently attach poly(ethylene glycol) (PEG) to the surface of theimmobilized enzyme. When conjugated to an enzyme, the PEG chains form afluid barrier that via steric effects blocks interactions with otherproteins. PEG-protein conjugates are readily synthesized by reactingactivated PEGs with functional groups such as amines or thiols on theside chain of amino acids. In the case of the modified HFMs, the CA canbe PEGylated before or after immobilization onto the fiber surface.Additionally, by blocking the adsorption of plasma adhesive proteinssuch as fibrinogen, PEGylation of the immobilized CA may preventthrombotic deposition on the fiber surface. As set forth in plateletactivation studies discussed below, the enzyme itself seems to assist inpreventing thrombotic deposition on the fiber surface. The adhesion ofplatelets, which can lead to acute thrombosis on blood-contactingbiomaterials, may potentially block diffusion of CO₂ through the HFMs.See, for example, Xu H., Kaar J L, Russell A J, Wagner W R.Characterizing the modification of surface proteins with poly(ethyleneglycol) to interrupt platelet adhesion. Biomaterals 2006; 27,3125-3135.)

The biocompatibility of CA-immobilized HFM was assessed using a plateletactivation assay. Whole sheep blood, containing 6 unit/mL of heparin,was incubated with CA-immobilized and unmodified HFM for 2 hrs with mildshaking at 37° C. At the end of the incubation step, the relativefraction of activated platelets in the blood was quantified by flowcytometry using the protein annexin V as a marker. Results of the assayindicated that the immobilization of CA significantly reduced plateletactivation (see FIG. 10A, p<0.05). Additionally, the degree of plateletdeposition onto the surface of the CA-immobilized and unmodified HFM wasobserved by SEM. Images of CA-immobilized HFM showed considerably lessplatelet deposition than in images of unmodified HFM (FIG. 10B).

The impact of immobilized CA on CO₂ removal from bovine blood wasdetermined using a model respiratory assist device comprised of a bundleof single layer CA-immobilized HFM similar to that illustrated in FIG.5. The blood contacting membrane area in the device was 83.57 cm². Freshblood (40 mL) was perfused around the fibers at a rate of 150 mL/minwhile the flow rate of sweep gas was set at 150 mL/min. A blood gasanalyzer (Radiometer America, ABL 555, Westlake, Ohio) was used tomonitor the amount of CO₂ in the blood every 2.5 min over the course of10 mins. The rate constant for CO₂ removal was determined by usingmodified CO₂ dissociation curve equation as follows:TCO₂ =apCO₂ ^(b)  (1)

where TCO₂ represents the total CO₂ (volume %) and pCO₂ is the partialpressure of dissolved CO₂ (mmHg).

A mass balance on the blood volume in the model device yields:d/dt[TCO₂ ]=kApCO₂  (2)

where k is the mass transfer coefficient and A is the membrane surfacearea.

Substituting into equation 1:abpCO₂ ^(b-1) d/dt[pCO₂ ]=kApCO₂  (3)

Integrating equation 3 results in:pCO₂(t)^(m) *−pCO₂(t ₀)^(m)*=K*t  (4)

where pCO₂(t) and pCO₂(t₀) represent pCO₂ values at designatedmonitoring time (0, 2.5, 5, 7.5, and 10 min) and zero time,respectively, m* is equal to b−1, and K* is equal to k·(b−1)A/a·b whichrepresents the CO₂ removal rate constant. The CO₂ removal rate constantis directly proportional to the mass transfer coefficient, k, dictatingCO₂ removal as per eq 2.

The non-optimized results of several studies showed that the rate of CO₂removal with CA-immobilized HFMs was improved by, for example, 10-30%compared to unmodified HFM (FIGS. 11A and 11B).

FIG. 12A illustrates an embodiment of a commercially availableartificial lung device 100 including a housing 110 into which HFMincluding immobilized CA of the present invention can be incorporated orretrofitted. The most common artificial lungs are the blood oxygenatorsused in cardiopulmonary bypass circuits, and examples include, but arenot limited to, the CAPIOX® SX available from Terumo CardiovascularSystems, the QUODROX® available from Jostra, and the AFFINITY®,available from Medtronic.

In addition to use in these commercially available oxygenators, HFMincluding immobilized CA of the present invention can also beincorporated into artificial lungs being developed as respiratory assistdevices. As used herein, the term artificial lung includes respiratoryassist devices. These devices include total artificial lungs (TALs)being developed as bridge to lung transplant devices as well asrespiratory assist devices being developed for acute lung failure oracute-on-chronic lung failure. FIGS. 12B and 12C illustrate anembodiment of a paracorporeal respiratory assist lung (PRAL). The deviceof FIGS. 9B and 9C uses hollow fiber membranes that are rotated toimprove gas exchange and also to pump blood.

Most blood oxygenators or respiratory assist devices are artificiallungs that include either microporous polypropylene hollow fibermembranes or, as in some design, silicone sheets. The general anatomy ofoxygenators and respiratory assist devices are similar between the twotypes of devices and are even similar between fiber membrane versussheet membrane devices despite the differing gas exchange surfaces.Referring to device 100 of FIG. 12A, blood enters oxygenator 100 throughan inlet port 120 and flows either along the outside of hollow fibers130 or the outside of the silicone sheet. The blood is then collected ina manifolded region, flows through a heat exchanger 140, and then exitsthe device through an outlet port 150. Water of other fluid port 144 canbe provided in connection with heat exchanger 140. The gas, which can bepure oxygen or a mixture of oxygen and room air, enters oxygenator 100through a gas inlet port 160, flows through the inside of the hollowfibers/silicone sheets 130, and exits the device via an outlet port 170.

Important design considerations in blood oxygenators include minimizingthe resistance to blood flow, reducing the priming volume, ensuring easydebubbling at setup, and minimizing blood activation andthrombogenicity. Most currently available blood oxygenators have fibermembranes with outer diameters of approximately 200-400 microns and wallthickness of approximately 20-50 microns, total membrane surface area ofapproximately 2-4 m², and blood priming volume of approximately 135-340ml. The hollow fibers are, for example, wound or matted within a hardplastic outer shell to produce fiber packing densities in the bundle ofapproximately 40-60%, and the arrangement of the fiber bundle and bloodflow patterns differ between devices. For example, fibers are helicallywound in the Medtronic AFFINITY NT oxygenator. Blood enters the devicethrough a central core channel and is then distributed radially throughthe fiber bundle. Fibers in the Jostra QUODROX oxygenator are, forexample, aligned so that blood flow is perpendicular to the gaspathways. Hollow fiber oxygenators with intraluminal blood flow havebeen designed but are used less often because of a generally unfavorablehigh resistance to blood flow and inferior gas exchange efficiency (gasexchange per membrane area). Respiratory assist devices may use lessfiber membrane surface area than commercial blood oxygenators eitherbecause gas exchange requirements are less or because the devicesincorporate special features that improve the rate of gas exchange perunit fiber surface area. An example is the rotating fiber bundle of PRAL200 of FIGS. 12B and 12C developed for acute respiratory support. PRAL200 includes an outer housing 210, a fiber bundle system 220 and astationary inner core 230. Another type of artificial lung beingdeveloped that can use HFMs with immobilized CA of the present inventionis an intravascular respiratory assist catheter. FIG. 12D illustrates anexample of such a device 300, which is, for example, designed forplacement within the large veins returning blood to the heart.Intravascular respiratory assist catheter 300 does not have plastichousings incorporating fiber bundles 310, as such fiber bundles 310 aredesigned to be floating within a blood vessel while manifolded viamanifolds 320 and connected through tubing 330 (via an externalconnector 340) to a gas supply source outside the body. An expandableballoon 350 is placed internal to fiber bundle 310. Designs can also usemixing impellers in the place of balloons to enhance gas exchange.

Silicone membrane oxygenators are often used in extracorporeal membraneoxygenation for respiratory support since plasma leakage does not occuras can occur in microporous hollow fiber oxygenators. A spiral-woundsilicone membrane oxygenator contains two silicone sheets sealed aroundthe edges, which are wound around a polycarbonate core. Gas flows withinthe sealed sheets and blood flows countercurrently between the spiralwraps. The surface area of silicone membrane oxygenators ranges fromapproximately 0.4 to 4.5 m² and the priming volumes range fromapproximately 90 to 665 ml. Because diffusion occurs across a nonporoussilicone sheet, the thickness of these sheets was reduced toapproximately 100-200 μm. Nevertheless, the gas exchange efficiency ofsilicone oxygenators is substantially below that of hollow fiberoxygenators. The Avecor 0800 silicone oxygenator, for example, has an O₂transfer efficiency of 88 ml/min/m² compared to 150 ml/min/m₂ for theAFFINITY hollow fiber device and 250-500 ml/min/m² for the respiratoryassist devices shown in FIGS. 12B and 12D. The resistance to blood flowis also higher in silicone sheet oxygenators than in hollow fiberoxygenators, and debubbling the sheet oxygenators can be more difficult.

As discussed above, CA can be immobilized on a wide variety of polymericmaterials, including, but not limited to, microporous hollow fibermembranes and silicone sheets as described above. Hollow fiber membranesin artificial lungs (blood oxygenators or respiratory assist devices)are typically made using polyolefin polymers, with polypropylene,polyethylene and polymethylpentene being commonly used materials. Thehollow fiber membranes are created by extrusion or other manufacturingprocesses to create a microporous fiber wall with sub-micron sized poresspanning the walls. These microporous fibers are adequate in commercialblood oxygenators for short term blood contact. In respiratory assistdevices and artificial lungs required for longer-term blood contact (>6hours), the microporous fibers can leak plasma into the fiber lumens andcreate a problem known as plasma wetting. To prevent or retard plasmawetting, composite or asymmetric microporous hollow fiber membranes havebeen developed. A composite HFM is a standard microporous hollow fibermembrane on which a thin coating of a nonporous (dense) polymer isapplied. FIGS. 13A and 13B illustrate cross section views of HFMs withand without a nonporous coating, respectively. FIGS. 13C and 13Dillustrate scanning electron micrographs of HFMs with and without anonporous coating, respectively. As the nonporous coating polymers needto be very gas permeable the coating is often made from siloxanepolymers. An asymmetric microporous hollow fiber membrane is one inwhich the fiber wall is fabricated so that the porosity varies acrossthe wall and goes to zero at the fiber surface. The pores are thenessentially sealed by the same material from which the fiber is made.FIG. 14 illustrates a scanning electron micrograph of an HFM fabric inwhich woven support threads are used to connect the hollow fibers. Onceagain, the CA immobilization techniques of the present invention can beused in connection with composite HFM, asymmetric HFM, HFM fabric,silicone sheets and many other polymeric and other surfaces.

FIG. 15 illustrates an idealized representation of a hollow fiber uponwhich carbonic anhydrase has been immobilized upon the outer surface. Asillustrated oxygen passes from the lumen of the hollow fiber to blood onthe exterior of the fiber and carbon dioxide passes from the blood onthe exterior of the hollow fiber to the lumen of the hollow fiber. FIG.16 illustrates scanning electron micrographs of microporous hollow fibermembranes suitable for use in artificial lungs wherein the walls of thefibers (right) contain submicron pores through which respiratory gasesdiffuse.

Such fibers and other materials upon which CA has been immobilized asdescribed herein can be used in other modules or devices designed toremove CO₂ from fluids (for example, liquids, gases and combinationsthereof) in which a component of the CO₂ is in the form of bicarbonateion. The liquids can also include CA in solution to facilitate theconversion of CO₂ to HCO₃ and HCO₃ ⁻ to CO₂. Several examples include,but are not limited to, liquids used in total liquid ventilation, inwhich lungs are ventilated with oxygen carrying liquids likeperfluorocarbons. These liquids are part of a closed circuit and, hence,CO₂ should be eliminated effectively from the liquids before they arepumped back into the lungs. Another potential application is in systemsusing peritoneal or gastric perfusion to provide respiratory support.Similar to liquids used in total liquid ventilation, the liquids used inperitoneal or gastric perfusion require an effective means to eliminateCO₂ before being pumped back into respective body compartments.

Further, it is not necessary that the CA be immobilized upon the surfacethrough which gas diffusion/flow takes place. In the case of removal ofCO₂ from blood, such a secondary surface upon which CA is immobilized ispreferably in close proximity to the surface (for example, HFM) throughwhich gas flow occurs. Preferably, the secondary surface is withinapproximately 50 μm of the gas diffusion membrane. More preferably, thesecondary surface is within approximately 10 to 30 μm of the gasdiffusion membrane. CA can, for example, be immobilized upon supportthreads that are used to connect the hollow fibers as illustrated inFIG. 14.

Experimental Examples

Materials. Carbonic anhydrase (CA) from bovine erythrocytes waspurchased from Sigma-Aldrich (St. Louis, Mo.) and used without furtherpurification. Polymethyl-pentane (PMP) hollow fiber membranes (Oxyplus,Type PMP 90/200, OD: 380 μm, ID: 200 μm) were obtained from MembranaGmbH (Wuppertal, Germany). All other reagents were purchased fromSigma-Aldrich (St. Louis, Mo.) and were of analytical grade or purer.

CA Immobilization. PMP HFMs (Oxyplus, Type PMP 90/200, OD: 380 μm, ID:200 μm) were used as the substrate for CA immobilization. Initially, theHFMs were treated by RFGD (GCM-250, March Plasma Systems, Concord,Calif., see FIG. 9L) using water as a plasma source. A range of plasmadischarge powers (25, 50, and 100 W) and treatment time (30, 60, 90, and180 s) were employed.

After RFGD treatment, the modified fibers were immersed in a 2 M Na₂CO₃solution (no pH adjustment). CNBr (1 g/mL in acetonitrile) was added tothe buffer solution to a final concentration of 100 mg/mL in which thefibers were incubated for 10 min with mild shaking The fibers weresubsequently washed extensively with ice-cold deionized water andcoupling buffer (0.1 M Na₂CO₃, pH 8.0). Conjugation of CA to theCNBr-activated fibers was initiated by adding the fibers to couplingbuffer containing 1 mg/mL CA. The reaction mixture was incubated for 3hr after which any loosely adsorbed CA was removed by washing threetimes with phosphate buffer (50 mM, pH 7.5).

CA Activity Assay. The catalytic activity of CA was assayed usingp-nitrophenyl acetate (p-NPA) as the substrate as described by Drevon etal. (2003). The mechanism of catalysis is believed to be the same forthe dehydration of HCO₃ ⁻¹ and hydrolysis of p-NPA. Pocker Y, SarkanenS. 1978. Carbonic anhydrase: structure, catalytic versatility, andinhibition. Adv Enzymol 47:149-274 and Pocker Y, Storm DR. 1968. Thecatalytic versatility of erythrocyte carbonic anhydrase. IV. Kineticstudies of the enzyme-catalyzed hydrolyses of p-nitrophenyl esters.Biochemistry 7:1202-1214. Briefly, p-NPA substrate dissolved inacetonitrile (40 μL, 40 mM) was added to different concentrations of CA(4 mL) prepared fresh in phosphate buffer (50 mM, pH 7.5). Enzymeactivity was measured spectrophotometrically using a Genesys 5 UVspectrophotometer (Thermo Spectronic, Somerset, N.J.) by monitoring thehydrolysis of p-NPA to p-nitrophenol (p-NP) at 412 nm. Absorbancemeasurements were recorded every 1.5 min over the course of 6 min andplotted as a function of time. The molar extinction coefficient of p-NP(11.69 cm⁻¹mM⁻¹) was measured and used to calculate enzyme activity. Oneactivity unit was defined as the amount of enzyme that generates 1 μmolp-NP per minute.

The activity of CA immobilized on fiber membranes was measured using amethod similar to that employed for assaying free CA. CA immobilizedHFMs were cut into 1-2 mm segments and placed in a beaker (20 mL volumesize) to which assay buffer (50 mM phosphate buffer, pH 7.5) was added(4 mL). The solution was mixed vigorously using a magnetic stirrer andthe reaction was initiated by addition of the substrate (40 μL). Tomeasure the absorbance, the solution was filtered and transferred to acuvette using a syringe (BD, Franklin Lakes, N.J.) equipped with asyringe filter (PCG Scientific, Gaithersburg, Md.). The absorbance wasmeasured at 3 min intervals over the course of 12 min.

Determination of Gas Permeance. The gas permeance of each fiber testedwas measured using the method of Eash et al. (2004). Briefly, each fibersample was fixed in nylon tubing. One end of the hollow fiber wasoccluded with glue and the other end of the hollow fiber was open to thegas outlet pathway. The tube inlet was connected to a gas source fromwhich the flow of either CO₂ or O₂ was controlled via a pressureregulator. Gas permeance across the fiber membrane was determined as afunction of the differential pressure between the gas inlet and outletextending from the lumen of the fibers using a pressure transducer(SenSym Inc., Milpitas, Calif.). Gas flow rate was measured using abubble flow meter (Supelco, Bellefonte, Pa.). Room temperature andatmosphere pressure was also recorded for the calculation of gaspermeance. Gas permeance across the hollow fiber membrane was calculatedusing the following equation:

$K = \frac{Q}{{S \cdot \Delta}\; P}$

where K represents the gas permeance (mL/s/cm²/cmHg), Q is the measuredgas flow rate (mL/s), S is the calculated surface area of fiber exposedto test gas (cm²), and ΔP is the differential pressure across the fiberwall (cmHg).

SEM Analysis of Hollow Fiber Membranes. RFGD plasma modified PMP HFMswere analyzed using SEM (JSM-6330F, JEOL, Peabody, M A) to characterizethe outer surface of the membranes. Prior to analysis, the specimenswere coated with a 3.5 nm gold/palladium layer to conduct electricityusing a sputter coater (Cressington Auto 108, Cressington, Watford,U.K.). The samples were then placed inside the vacuum column of theinstrument through an air-tight door. Images were taken at 100,000 timesmagnification using an accelerating voltage of 10 kV.

Assessment of CO₂ Removal in a Mini-Lung Module. Mini-lung modules werefabricated containing either modified or unmodified HFMs to compare theimpact of CA-immobilization on the efficiency of CO₂ removal. Briefly,unmodified or CA-immobilized fibers (60 fibers, 10 cm) were insertedinto a tubular module and fixed at both ends with an epoxy adhesive. Theloose ends of the fibers at the fixing point were cut off Inlet andoutlet ports on the module allowed for the continuous flow of sodiumbicarbonate buffer (2 mg/mL, pH 7.5), which was controlled by aperistaltic pump (MasterFlex C/L, Cole Parmer, Vernon Hills, Ill.). Theflow of buffer was circulated in a closed loop with a maximum capacityof 10 mL such that the amount of CO₂ in the apparatus was tightlycontrolled. Two ¼″×¼″ single luer locks (Qosina, Edgewood, N.Y.) wereattached to the ends of the module serving as ports for the flow ofsweep gas through the lumen of the fibers. Using tygon tubing (ID:0.1099″), one end of the module was connected to a gas cylinder fromwhich the flow of gas was controlled with a pressure regulator while theother end of the module was connected to a bubble flow meter. Theresidual concentration of CO₂ in the circulating buffer was monitoredpotentiometrically over time using a Analytical Sensor Instruments(Sugar Land, Tex.) CO 35 model CO₂ electrode and a Corning (Corning,N.Y.) 314 pH/Temperature Plus pH/mV meter (see FIG. 5).

In all experiments, pure oxygen was used as the sweep gas. The flow rateof the sodium bicarbonate buffer and sweep gas were set at 10 and 30mL/min respectively. Fibers with a range of immobilized CA activity (0,0.20, 0.25, 0.30 U) were employed. Experiments in which free CA (0,0.25, 0.50, 1.00 U) was added to the module containing unmodified fiberswere also performed. For these experiments, free enzyme was injecteddirectly into the outer shell compartment of the mini-lung module aftera stable CO₂ measurement was reached. To accurately measure thereduction of CO₂ in the closed loop system, total CO₂ rather than onlydissolved CO₂, which is in equilibrium with the conjugate bicarbonatespecies (HCO₃ ⁻) was measured. Samples were removed every 10 mins over aperiod of 30 mins and diluted ten-fold in a dilute acid solution. Theaddition of acid ensured the pH of the sample was less than 5 at whichpoint all bicarbonate is converted into CO₂.

Additionally, a desorption test of the enzyme was performed to evaluatethe stability of CA attachment on the HFM. 10 mL of 50 mM phosphatebuffer (pH 7.5) was circulated for 30 min in the shell compartment ofthe mini-lung module containing CA immobilized fibers, and then thesolution was harvested for enzyme activity assay. We repeated the aboveprocedures 3 times using the same module. The enzyme activities in thesolutions were analyzed for the above samples.

The foregoing description and accompanying drawings set forth thepreferred embodiments of the invention at the present time. Variousmodifications, additions and alternative designs will, of course, becomeapparent to those skilled in the art in light of the foregoing teachingswithout departing from the scope of the invention. The scope of theinvention is indicated by the following claims rather than by theforegoing description. All changes and variations that fall within themeaning and range of equivalency of the claims are to be embraced withintheir scope.

1. A device for removal of at least a portion of carbon dioxide from anaqueous fluid, comprising: at least one structure through which carbondioxide can pass to be removed from the fluid, the structure comprisingan outer portion that is nonporous and gas permeable and an innerportion that is porous, the outer portion comprising active carbonicanhydrase immobilized thereon, the outer portion being positioned to becontacted with the fluid such that the immobilized carbonic anhydrasecomes into contact with the fluid.
 2. The device of claim 1 wherein thefluid is blood and at least the outer portion comprises a polymericmaterial.
 3. The device of claim 2 wherein the carbonic anhydrase isimmobilized on the polymeric material via adsorption, covalent bonding,ionic bonding or chelation.
 4. The device of claim 2 wherein thecarbonic anhydrase is covalently attached to the polymeric material. 5.The device of claim 4 wherein the inner portion is microporous such thatcarbon dioxide can pass therethrough.
 6. The device of claim 5 whereinthe inner portion is hydrophobic.
 7. The device of claim 6 wherein thethe inner portion comprises a polymeric material.
 8. The device of claim5 wherein the carbonic anhydrase is covalently attached to the polymericmaterial of the outer portion of a hollow fiber.
 9. The device of claim8 wherein an interior lumen of the hollow fiber is adapted to haveoxygen flow therethrough, the hollow fiber being adapted to pass oxygeninto the blood while carbon dioxide passes from the blood to theinterior lumen of the hollow fiber.
 10. The device of claim 5 whereinthe nonporous and gas permeable outer portion forms an exterior surfaceof a microporous polymeric hollow fiber.
 11. The device of claim 9comprising a plurality of hollow fibers.
 12. The device of claim 1wherein the outer portion comprises CO₂-permeable silicone.
 13. Thedevice of claim 1 wherein the aqueous fluid comprise blood and thedevice further comprising free carbonic anhydrase to contact the blood.14. The device of claim 1 wherein carbon dioxide is present in the fluidin the form of bicarbonate ion.
 15. The device of claim 1 wherein thedevice is a component of a total liquid ventilation circuit adapted tobe connected to lungs.
 16. The device of claim 1 wherein the fluid is anoxygenated perfluorocarbon.
 17. The device of claim 1 wherein the deviceuses peritoneal or gastric perfusion to provide respiratory support. 18.The device of claim 1 wherein the device is a component of an artificiallung.
 19. A method for removal of at least a portion of carbon dioxidefrom an aqueous fluid, comprising: placing at least one membrane throughwhich carbon dioxide can pass to be removed from the fluid in contactwith the fluid, the membrane comprising immobilized active carbonicanhydrase on or in the vicinity of a nonporous and gas permeable outersurface thereof such that the immobilized active carbonic anhydrasecomes into contact with the fluid.
 20. A device for removal of at leasta portion of chemical entity from a fluid, comprising: at least onemembrane through which the chemical entity can pass as a gas to beremoved from the fluid and immobilized active enzyme immobilize on anonporous and gas permeable outer surface of the membrane to becontacted with the fluid such that the immobilized enzyme comes intocontact with the fluid, and wherein the chemical entity isinter-convertible within the fluid to at least one other chemicalentity, the enzyme being adapted to catalyze a reaction of the otherchemical entity to the chemical entity.